An improperly balanced window sash will not stay wherever it is set it will "hop" if overbalanced and "drop" if underbalanced. The remedy is to use a different balance system that more accurately matches the sash weight. A difficulty with this is that a large inventory of balance systems with slightly different balance forces must be available to match the correspondingly large variety of sash sizes and weights, and this adds to the expense of balancing sashes.
Although relief for this problem has been devised for some types of window balance systems (see for example U.S. Pat. No. 4,517,766), no relief has been proposed for balance systems of the block and tackle type. These involve a strong spring with a large spring force and a block and tackle system using pulleys and a cord to reduce and spread the spring force over the distance traveled by the sash (see for example U.S. Pat. No. 3,358,404). To help reduce the size inventory necessary for block and tackle type window balance systems, I have devised a variable friction device that can change the friction of a single block and tackle window balance system so that it accommodates a wider range of sash weights. My variable friction device is also made simple, effective, adjustable after installation with a sash, and inexpensive enough to scarcely increase the cost of a block and tackle window balance system.